Exhaust gas from diesel engines and lean-burn gasoline engines includes particulates (particulate matter) composed mostly of carbon. To collect the particulates, a filter is provided in an exhaust gas passage. When the amount of the collected particulates increases, the particulates are combusted and removed from the filter for the regeneration of the filter. In order to promote the combustion of the particulates, a catalyst is loaded on the filter body.
The above catalyst includes a typical Pt/alumina catalyst having Pt loaded on alumina. PATENT DOCUMENT 1 discloses a technique which involves loading a granular material having a predetermined particle size distribution on a filter body, and then impregnating the granular material with a Pt solution to load a granular material on the filter body. The document cites that examples of the granular material include γ-alumina and ceria, and the granular material may contain alkali earth metal allowing the catalyst to absorb and reduce NOx.